In my case I can remember thinking that some mischievous leprechauns, hiding
underground nearby of course, must be the ones controlling traffic lights-how else
to explain the lights’ seemingly capricious nature?
god
The notion that adults were always confident and always knew what they were doing.
The idea that being an adult was fun because there’s nobody to make you do the stuff you don’t want to do!
When my sister was little, she thought satelites were “saddle lights” and they were like headlights that cowboys used! 
There was a silly notion that I had both formulated and was disabused of within moments of each other. When I was 5 or 6, some friends and I decided to climb a tree that was in the school’s rather large yard. I was in the lead, and had made it most of the way up when I looked down and saw drops of blood falling on to the branches below.
“Hey!” I called to my friends below. “The tree is bleeding! The tree is bleeding!” The idea that trees contained blood was just as surprising to me as the fact that it was bleeding in the first place. My friends, however, were looking at me funny, like there was something they were afraid to tell me. Did I do something bad to the tree? Was I going to get in trouble? My hair felt wet. I brought my hand up to the crown of my head. It came away slick with blood. The tree was not, in fact, bleeding. I was. I didn’t feel a thing, either.
It was a bit of a let-down, really. I thought I’d discovered something amazing, like some Canadian Blood Tree or something. All I could say to the reality, however, was a sheepish “Oh.”
Earned me a trip to the nurse’s station. Evidently I’d bumped my head on a particularly vicious thorn in said tree. I still didn’t feel anything except for the bitterly cold ice pack that I really didn’t think I needed.
I was absolutely convinced for a few years that kids are kids and grownups are grownups, but that kids don’t grow up to become grownups, and everyone who said so was part of a Vast Conspiracy. I could find no other way to explain why none of the adults in my life seemed to “get” how hard being a kid was - they all talked about how youth was wasted on the young and what a wonderful idyllic time childhood was and I was just like, “Are you kidding me? Obviously, you’ve never been a kid, 'cause it pretty much sucks ass!”
More recently, I’ve been disabused of two magazine marketing notions. About three years ago, I finally realized that one can pursue a career in a field you don’t love. Sounds obvious, I know, but I had totally bought into the articles on Mrs. Fields, Oprah, Donald Trump, et al where they talked about finding what you love and the rest will just fall into place and you’ll be a millionaire. I’m so glad that worked out for them, but for me it means I’m 32 and going back to school because I never figured out what I had a passion for. Now I’ve found something I think I can stand (nursing) and that will have to do.
Just this week, a Doper helped disabuse me of the other one, which is that if you get organized and follow a routine, icky jobs like keeping your home clean, dieting and exercise become easy and can even be enjoyable. Ha, very ha.
Oh, god, that was a hard one. When I was about 19, I was sobbing on my mother’s shoulder, and I just asked her when I’d stop being scared and uncertain all the time. She was silent for a minute, then just said, “I’ll let you know when it happens to me.”
If you were a good and giving person, people appreciated it and you got ahead.
In reality: You’re a good and giving person, and people appreciate it. They’ll continue taking as long as they can, and it doesn’t help you get a single step ahead.
Well, I’m not sure if this is what you mean, but I no longer believe that the Partridge family might, just might, live in the radio of my mom’s VW Beetle.
That everybody grows old and dies peacefully in their sleep, except for very rare instances where they die in horrible accidents.
That doctors all grew up wanting to be doctors more than anything, because they feel driven to help people. Also that doctors always knew best.
I’m 45, and clearly remember that, at least for me, childhood definitely sucked ass! I do try to convince my kids every once in a while that being an adult isn’t as much fun as it may seem, but I never try to convince them that being a child is the most wonderful thing in the world!